Horse racing's Triple Crown winners as American Pharoah attempts to win it at Belmont Stakes in 2015

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But only 11 horses have won horse racing's Triple Crown: the Kentucky Derby, the Preakness and the Belmont Stakes in the same year.

On Saturday, June 6, at the Belmont Park, American Pharoah attempts to become the 12th horse to accomplish one of the toughest feats in sports.

Here's a look back at those 11 horses who won the Triple Crown.

1978: AFFIRMED

Affirmed on the inside, with jockey Steve Cauthen up, wins the Belmont Stakes. The great-grandson of Triple Crown winner Man o'War, Affirmed is the most recent winner of horse racing's holy grail. Following retirement, Affirmed sired over 80 stakes winners that earned in excess of $ 44 million through 2004.

1977: SEATTLE SLEW

Seattle Slew blew away the field in The Belmont Stakes winning by four lengths to seize the Triple Crown. Seattle Slew was not expected to be a great thoroughbred and was purchased for the bargain price of $17,500. He died in 2002, the last of the living Triple Crown winners.

1973: SECRETARIAT

The first Triple Crown winner since 1948, Secretariat is arguably the greatest horse of the 20th century. He set track records in all three Triple Crown races including a world record in The Belmont Stakes. Secretariat came from last to first to win the Kentucky Derby and did the same at the first turn of the Preakness. Secretariat entered The Belmont Stakes a 1-10 favorite and delivered, winning by an amazing 31 lengths.

1948: CITATION

The horse won 19 of 20 races that year including 16 in a row. In the horse's four-year career, Citation amassed 27 wins and two places in 29 starts. At 6 years old, Citation's victory in The Gold Cup made him the first horse to eclipse $1 million in career earnings. Citation sired Preakness winner Fabius.

1946: ASSAULT

Assault won the Kentucky Derby by eight lengths, the Preakness by a neck and the Belmont by three lengths over rival Natchez. Assault was sired by Derby and Preakness winner, Bold Venture. Nicknamed the "Club-footed Comet" Assault overcame a career plagued with injuries and illnesses to win horse racing's Crown Jewels. Assault, the only Texas-bred Triple Crown winner, won the Derby by a still standing record of eight lengths.

1943: COUNT FLEET

Count Fleet was sired by Kentucky Derby winner Reigh Count and owned by the wife of John D. Hertz, who founded the rental car company bearing his name. Count Fleet won 16 of 21 career starts including all six races entered in 1943 and never finished out of the money. The horse won The Belmont Stakes by 25 lengths, a record which stood until 1973.

1941: WHIRLAWAY

In addition to winning the Triple Crown, Whirlaway was twice named Horse of the Year and became the first horse to earn $500,000, retiring as the sport's top money earner.

1937: WAR ADMIRAL

War Admiral was sired by Man O' War. War Admiral was significantly smaller than his father though as his size was incorrectly portrayed in the movie, Seabiscuit. In the Belmont Stakes, War Admiral overcame an early stumble and injury to his front hoof to win the race. He won the Derby by eight lengths.

1935: OMAHA

Sired by Triple Crown winner Gallant Fox, Omaha won six of nine races entered in 1935, including the Triple Crown. Omaha is buried at the entrance to the Nebraska racecourse Ak-Sar-Ben. Despite winning the Triple Crown in 1935, Omaha failed to win the Eclipse Award for Horse of the Year as that honor went to fellow hall of famer, Discovery.

1930: GALLANT FOX

Jockey Earl Sande came out of retirement to ride Gallant Fox to the Triple Crown. In 1930, he won the Wood Memorial before winning the Preakness, Derby and Belmont. The term "Triple Crown" was first used by The New York Times to describe Gallant Fox's victories. Known as "the red-headed horse" for it's blood red color, Gallant Fox is the only Triple Crown winner to sire another winner, Omaha.

1919: SIR BARTON

After posting no wins as a 2-year-old, Sir Barton became horse racing's first Triple Crown winner the very next year. He won the Kentucky Derby by five lengths, the Preakness by four and the Belmont by five. In between the Preakness and Belmont, he won the Withers Stakes. In 1920, Sir Barton, bothered by sore hooves, lost a match race to the legendary Man O' War at Kenilworth Park.